Can a solar eclipse really blind you?

People in the western United States, Pacific and parts of Asia will have the chance to see a partial solar eclipse on Sunday (May 20). While it may be tempting to brush off warnings against looking up at this eclipse bare-eyed, don't: The light of an eclipse really can damage your eyes — though warnings of total blindness are likely overstated.

The condition is called solar retinopathy, and it occurs when bright light from the sun floods the retina on the back of the eyeball. The retina is home to the light-sensing cells that make vision possible. When they're over-stimulated by sunlight, they release a flood of communication chemicals that can damage the retina. This damage is often painless, so people don't realize what they're doing to their vision.

Solar retinopathy can be caused by staring at the sun (regardless of its phase), but few people can stand to look directly at our nearest star for very long without pain. It does happen occasionally — medical journals record cases in which people high on drugs have stared at the sun for long periods of time, causing serious damage. Adherents of sun-worshipping religious sects are also victims. In 1988, for example, Italian ophthalmologists treated 66 people for solar retinopathy after a sun-staring ritual. [ Gallery: Our Amazing Sun ]

But during a solar eclipse, more people are at risk. With the sun partially covered, it's comfortable to stare, and protective reflexes like blinking and pupil contraction are a lot less likely to kick in than on a normal day.

Early observers of astronomy sometimes found out about solar retinopathy the hard way. Thomas Harriot, who observed sunspots in 1610 but did not publish his discovery, once wrote in 1612 that after viewing the sun his "sight was dim for an hour." Oxford astronomer John Greaves was once quoted as saying that after sun observations, he saw afterimages that looked like a flock of crows in his vision. In the most famous case of all, Isaac Newton tried looking at the sun in a mirror, essentially blinding himself for three days and experiencing afterimages for months.

Scientists don't have a good bead on the prevalence of eye damage after a solar eclipse. In one study, conducted in 1999 after a solar eclipse visible in Europe, 45 patients with possible solar retinopathy showed up at an eye clinic in Leicester in the United Kingdom after viewing the eclipse. Forty were confirmed to have some sort of damage or symptoms; five of those had visible changes in their retina.

Twenty of the patients reported eye pain, while another 20 reported problems with vision. Of the latter group, 12 reported that their sight had returned to normal seven months later, but four could still see the ghosts of the damage in their visual field, such as a crescent-shaped spot visible in dim light. [ Gallery of Visual Illusions ]

"Our series demonstrates that, contrary to popular belief, the majority of people with eclipse retinopathy are not totally blinded," the researchers wrote in 2001 in the journal The Lancet. However, they warned, earlier post-eclipse studies had turned up more severe problems in patients, suggesting that widespread media warnings not to look at the eclipsing sun may have prevented more damage during recent eclipses.

Research also suggests that while a lot of the damage may heal, some may be permanent. One 1995 study followed 58 patients who sustained eye damage after viewing a 1976 eclipse in Turkey. Healing occurred during the first month after the eclipse, the researchers reported in the journal Graefe's Archive for Clinical and Experimental Ophthalmology, but by 18 months, whatever damage still remained was permanent up to 15 years later.

So, while it might be tough to go totally blind by looking at an eclipse, doing so without proper protection could leave a long-lasting stain on your vision. The only safe way to view an eclipse, according to NASA, is to use specially designed sun filters, often available at telescope stores, or to wear No. 14 welder's glasses, available at welding specialty stores. Pinhole viewers — essentially a hole in a piece of cardboard or paper — can also be used to view the eclipse indirectly by casting a shadow of the sun on the ground or on a screen.